Heat Treatment Basics part-0

Iron Carbon diagram

Scope:

In this article, the basic concepts of heat treatment will be discussed to give the reader an overview of previous knowledge of heat treatment processes.

Why Part-0:

Well, readers may be wondering why this part or rather an article is named zero. Just as there are four laws in thermodynamics (the study of particles in motion based on its temperature), the first is the zero law and the last is the third law, the author follows a similar methodology to cover the basics of heat treatment in four parts.

Definition:

Classification of the heat treatment process:

1. Annealing.

(a). Relieves stress.

(b). Declaration of procedure

(c). Annealing.

(d). Full annealing.

2. Normal.

3. Hardening (by mitigation).

4. Tempering.

5. Martempering.

6. Austempering.

7. Maraging.

Purpose of heat treatment:

👉Relief due to internal stress.

👉Hard and strong metal.

👉Improve mechanization.

👉Change grain size.

👉Softening metals for further work.

👉Improve flexibility and toughness.

👉Increase, heat, wear, and corrosion resistance.

👉Improvement in electrical and magnetic properties (See note-1).

👉Circulating small particles by diffusion (See note-2).

Note - 1: Very limited information applicable to heat-curable rain-hardened aluminum alloys is available.

Note - 2: Here annealing through diffusion is employed to remove any structural non-homogeneity such as dendrites, columnar grains, and chemical inhomogeneities that promote brittleness, reducing steel ductility and toughness.

Principle of heat treatment:

Possible via eutectoid reaction [This is an isothermal reversible reaction in which a solid phase {austenite} is converted into two {pearlite-ferrite + cementite} or more intensely mixed solids when cooled, the temperature of the eutectoid steel. Is 723 ° C]. Iron-carbon system (figure 1 shows below).

👉 Heat treatment of steel involves alteration or decomposition of austenite.

👉The transformed product develops a range of useful physical and mechanical properties.

👉The cooling rate determines whether the change from austenite will produce perlite or martensite.

👉 One of the effective elements for heat treatment must be soluble in a solid-state (alloy) in varying amounts under different conditions.

👉 The principle of heat treatment is around the theory that an alloy transforms into microstructure when it is heated above a certain temperature in the experience and when cooling to room temperature it undergoes changes in the microstructure again.

👉Slow cooling above a critical threshold will produce perlitic microstructure

👉 Fast cooling will boost martensitic microstructure

Steps of heat treatment processes:

👉 Heating a metal or alloy to determine the temperature.

👉 Hold at that temperature for a sufficient period to allow the necessary changes to occur.

👉 With the change in the nature, form, size, and distribution of the micro components cooled at the rate required to achieve the desired properties.

1. Annealing:

Definition:

It is a process of heating metal in a metastable state, at a temperature that will overcome instability, and then cooling so that the room temperature structure is stable.

An objective:

👉Microstructure stabilization (complete annealing)

👉Refining and homogenizing microstructure

👉Reduce hardness

👉Improvement in machinability, cold working characteristics, mechanical, physical, electrical, and magnetic properties

👉Removal of residual stresses and gases

👉Production of desired microstructure

 (a). Stress Relief (Recovery):

👉It relieves stress produced by casting, quenching, machining, cold work, welding, etc. The same applies to iron and non-material.

👉This is often desirable when casting is liable to change dimensions to a deleterious degree during machining or use. If unrelated stresses may cause warpage or failure of casting.

👉Thermal stress relief requires the casting to be heated to a temperature at which the relaxation of the elastic stress to the elastic stress is brought about by the plastic strain.

👉It does not affect the metallurgical structure of the casting but is essentially a creep; The temperature required for SR is 0.3 to 0.4 times the melting point of cast metal or alloy.

 (b). Process annealing (Sub-critical):

👉Usually applied to counter the effects of cold working, and to allow further cold work in the form of weld wire drawing.

👉Iron alloys are heated below the lower transformation temperature range (550–650 ° C), held at that temperature, and then cooled in air.

👉It is associated with only partial crystallization of deformed ferrite.

👉It does not involve any phase change and ferrite and cementite are present in the structure throughout the process.

Figure 2 below shows the various heat treatment processes.

(c). Spheroids Annealing

 👉In this type, the steel is subjected to a selected temperature, usually produced in a spherical or spherical form within or near the transformation limit.

Carbide in steel. See the figure below;

 An objective:

(i). Improves mechanization and surface finish during machining.

(ii). Facilitates subsequent cold working operations.

(iv). Some of the softener tool steel and air-hardening alloy steels.

Aspherical steel has low hardness and tensile strength and has a higher increase and decrease relative to the area than steel subjected to normal annealing. (Temporary range: 650–700 ° C).

(d). Full Annealing:

👉Declaration of an iron alloy by austenitizing and then cooling slowly through the transformation range.

👉The austenitizing temperature for hypo eutectoid steels is usually between 723 ° C and 910 ° C; And for hyperechoic steels, austenitizing temperatures between 723 ° C and 1130 ° C (refer to 1 above). These include:

(a) Heating steel to the appropriate annealing temperature in the austenitic zone;

(b) Holding a steel object at that temperature for a certain time depending on its thickness or diameter (about 2.5 to 3 min / mm thickness) so that it becomes completely austenitic; and then

(c) Cooling the steel object very slowly through the change range,

(d) Preferably in a furnace or any good heat-insulating material, until the object attains a low temperature.

The slow cooling associated with full annealing enables austenite to freeze to a lesser degree of supercoiling to form...

A pearlite + ferrite structure in hypo eutectoid steels;

 A Pearl site + Cementite Structure in Hyperutectoid Steels.

 An objective:

(a) Refine grain

(b) Remove strains

(c) Improvement - softness, machinability, formulation, electrical and magnetic properties

2. Normalization:

It has heating steel to about 40–50 ° C above its upper critical temperature and, if necessary, holding it at that temperature for a short time and then cooling the air at room temperature.

The type of structure obtained by normalization will depend largely on the thickness of the cross-section as it will affect the rate of cooling.

Microstructures consisting of ferrite and perlite are produced for hypo eutectoid steels.

 For eutectoid steels, the microstructure is simply perlite and it is perlite and cementite for hypo steroid steels.

 An objective:

(a). Forms a uniform structure.

(a). Refines the shape of the steel grain, which is added during rolling and forging.

(a). Reduce internal stress.

(a). Hyperutectoid terminates carbide networks at grain boundaries of steels.

3. Hardened by quenching:

It is the heat treatment of steel that increases its hardness by quenching (and tempering). The maximum% increase in hardness is achieved by quenching if they are between 0.35% and 0.60% carbon.

An objective:

(a). To resist hardened steel wear.

(b). Enables steel to cut other metals.

(c). Improve strength, toughness, and flexibility.

(d). Develops the best combination of strength and notch-flexibility.

In this type of steel, enough carbon (0.35% to 0.70%) is heated over 30 ° - 50 ° C, A3 line, at that temperature it is kept in a cross-section of 15–30 min per 25 mm. And then rapidly cooled or quenched in a suitable medium to produce the desired rate of cold and hardened steel.

4. Tempering:

The quenching produces hardening and masonite and is marshynet, very hard, and brittle to maintain; This can lead to cracks and deformation making the steel useless for service.

 In addition, the anulome-inverse is an unstable phase and the amplitudes may change as it changes over time. So it is necessary to temper the steel after quenching below the low critical temperature (A1).

Tempering Requirements:

(a). Heating hardened steel below A1 (low critical temperature).

(b). Holding for 3-5 minutes for each mm of thickness/diameter.

(c). Slow except in steels susceptible to cooling or cooling brittleness in steel.

Essentially the tempering reaction can be considered as the carbon atoms changes in the precipitated carbonate from increasing size carbide particles.

An objective:

(a). Relieve residual stress.

(b). Improve flexibility and toughness.

(c). Increase% elongation.

The figure below shows a typical quenched and tempering cycle:

Quenching Hardening and Tempering

Classification of tempering:

1. Low-temperature tempering:

1. Medium temperature tempering.

1. High-temperature tempering.

1. Low-temperature tempering.

👉It is carried out in a temperature range of 150 to 250° C.

👉Internal stresses are reduced, toughness, and ductility improved without affecting hardness. The structure is martensitic.

👉It is applied to cutting tools of carbon steels, low alloy steels, and for surface hardening and carburization.

2. Medium temperature tempering:

👉It is carried out in a temperature range of 350 to 450° C.

👉Develops a troostite structure.

👉Hardness and strength decrease while % elongation and ductility increase.

👉It imparts steel with the highest elastic limit with sufficient toughness.

👉Applied to coil springs, laminated springs, hammers, chisels, etc.

3. High-temperature tempering:

👉It is carried out in a temperature range of 500 to 650° C.

👉Develops a sorbate structure.

👉Eliminates internal stresses completely.

👉It imparts high ductility in conjunction with adequate hardness.

👉Applied to connect rods, shafts, gears, etc.

5. Martempering:

Heated above the critical range to make it all austenite, then quenched into a salt bath maintained at a temperature above the Ms and is held at this temperature long enough until the temperature is uniform across the section of the workpiece without transformation to austenite and subsequently cooling the workpiece in air through the martensite range, in turn resulting in martensite with a minimum of stresses, distortion and cracking which can be further tempered to increase ductility.

In practice, to utilize the benefits of martempering, alloying elements are added to steel. Otherwise, the critical cooling rate is too fast and the benefits of the martensite hardness can not be realized in parts that are large or even medium in size. Refer below the figure for martempering and Austempering;

Austempering and Martempering,

6. Austempering

👉 It consists of heating the steel to form all the ascent above the critical limit, then quenching in a salt bath or bathing in the main range (205 to 425 ° C) at a critical cooling rate. The piece remains in the bath until austenite is completely transformed into benefits and is then allowed to cool to room temperature, the rate is irreversible.

👉 It is a type of constrained mitigation that forms benefits instead of martensite.

👉 The formed steels are harder and more ductile than tempered martensite steel with hard tempering and tensile strength.

👉 The major limitation in size is the restriction for relatively thin sections so that the entire piece can quickly achieve the quenching bath temperature.

👉 For steels with higher hardness, larger sections may be used.

👉 Greater ductility and toughness with higher hardness.

👉 Less deformation and risk of quenching cracks.

7. Margin (Martensitic + Aging):

👉 Margin steels achieve high strength and ductility in conjunction with a martensitic type of hardening of a basic iron-nickel alloy, which hardens the age of martensite.

👉 In other words, these steels achieve ultra-high-strength when aged under martensitic conditions. (Eg 18Ni Maraj 250).

👉 Precipitation within the martensite is produced by tempering for 3 hours. To achieve high notched toughness and ultra-high tensile properties at 480 ° C.

Application:

Rocket and missile cases, aircraft structural parts, mortar and rifle tubing, hot extrusion dies, etc.

He has the following properties:

👉 Strength, flexibility, and toughness.

👉 Deformity.

👉 Good mechanization.

👉 Good weldability.

 (Note: This article is intended to give readers a general guideline, and will not be considered as an alternative to the code. Please read the relevant code and its sections for complete terms and conditions).